Migrate to hm managed theme switch

hm-modules/theme/default.nix

@@ -1,4 +1,4 @@
-{ config, lib, pkgs, ... }:
+{ config, lib, pkgs, options, ... }:
 
 with lib;
 
@@ -18,6 +18,35 @@ let
     };
   };
 
+  # Generate all theme-variant combinations
+  themeVariants = concatMapAttrs (name: theme: {
+    "${name}-dark" = { scheme = theme.dark; polarity = "dark"; };
+    "${name}-light" = { scheme = theme.light; polarity = "light"; };
+  }) cfg.themes;
+
+  # Base is gruvbox-dark, everything else is a specialisation
+  baseVariant = "gruvbox-dark";
+  specialisationVariants = filterAttrs (name: _: name != baseVariant) themeVariants;
+
+  availableVariants = concatStringsSep " " (attrNames themeVariants);
+
+  # Userspace theme switcher: activates a pre-built Home Manager
+  # specialisation generation. No sudo, no OS rebuild — just runs the
+  # specialisation's activation script in the base HM generation.
+  #
+  # The script logic lives in ./theme-switch.sh (plain shell, no Nix
+  # escaping); we only inject the two config-derived values it needs.
+  theme-switch = pkgs.writeShellScriptBin "theme-switch" ''
+    baseVariant="${baseVariant}"
+    availableVariants="${availableVariants}"
+    ${builtins.readFile ./theme-switch.sh}
+  '';
+
+  # Stylix is only declared on hosts that import its Home Manager module
+  # (e.g. `fujin`). Servers (susano, izanagi, amaterasu, ...) don't, so
+  # `stylix` isn't declared there. We probe option *declarations* (not
+  # config values) to avoid an evaluation cycle.
+  hasStylix = options ? stylix;
 in {
   options.dov.dynamic-theme = {
     enable = mkEnableOption "dynamic theme switching";
@@ -40,12 +69,35 @@ in {
     };
   };
 
-  config = mkIf cfg.enable {
-    # HM-level stylix config inherits from NixOS level
-    # Specialisations are handled by NixOS module
-    stylix = {
-      enable = true;
-      autoEnable = true;
-    };
-  };
+  config = mkIf cfg.enable (
+    {
+      home.packages = [ theme-switch ];
+    }
+    # Only reference the `stylix` option where it is actually declared.
+    # `mkIf false` is NOT enough: the module system still rejects a
+    # definition (even a disabled one) for an option that does not exist,
+    # which breaks every host that doesn't import stylix. `optionalAttrs`
+    # removes the key structurally, so non-stylix hosts evaluate cleanly.
+    // optionalAttrs hasStylix {
+      stylix = {
+        enable = true;
+        autoEnable = true;
+        # Base scheme/polarity; overridable per specialisation below.
+        base16Scheme = mkDefault themeVariants.${baseVariant}.scheme;
+        polarity = mkDefault themeVariants.${baseVariant}.polarity;
+      };
+
+      # Generate Home Manager specialisations for every non-base variant.
+      # Each is pre-built into the nix store during the single rebuild and
+      # activated userspace via `theme-switch` (no sudo, no OS rebuild).
+      specialisation = mapAttrs (name: variant: {
+        configuration = {
+          stylix = {
+            base16Scheme = mkForce variant.scheme;
+            polarity = mkForce variant.polarity;
+          };
+        };
+      }) specialisationVariants;
+    }
+  );
 }